The invention relates to methods for making various metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, nickel aluminides, titanium and titanium aluminide alloys, zirconium base alloys into engineering components by melting of the alloys in a vacuum or under a low partial pressure of inert gas and subsequent casting of the melt under vacuum or under a low pressure of inert gas in molds machined from fine grained high density, high strength isotropic graphite wherein the mold cavity is uniformly coated with pyrolytic graphite.
A. Investment Casting
If a small casting, from xc2xd oz to 20 lb (14 g to 9.1 kg (mass)) or today even over 100 lb (45 kg), with fine detail and accurate dimensions is needed, lost wax investment casting is considered. This process is used to make jet engine components, fuel pump parts, levers, nozzles, valves, cams, medical equipment, and many other machine and device parts. The investment casting is especially valuable for casting difficult-to-machine metals such as superalloys, stainless steel, high-nickel alloys and titanium alloys.
The process is slow and is one of the most expensive casting processes. If a design is changed, it may require expensive alterations to a metal die (as it would in die casting also).
Preparation of investment casting molds requires operation of several equipment involving many manual processing steps such as the following.
(a) Fabrication of wax patterns via injection molding equipment, (b) manual assembly of wax patterns, (c) dipping wax patterns in six to nine different alumina or zirconia ceramic slurries contained in large vats, (d) dewaxing the molds in autoclave, and (e) preheating the molds to 2000xc2x0 F. in a furnace prior to vacuum casting.
Wax injection pattern dies are expensive depending on the intricacy of the part. Lead time of six to twelve months for the wax injection die is common in the industry. Defects often occur in wax patterns due to human errors during fabrication. These defects are frequently repaired manually, which is a time consuming process.
Ceramic molds are cracked frequently during dewaxing, that leaves a positive impression on the castings, which requires manual repair.
Ceramic facecoat applied after the first dip of the wax patterns in the ceramic slurry tends to spall or crack which often get trapped as undesirable inclusions in the final castings. Ceramic facecoat would react with rare earth elements in the superalloy, such as yttrium, cerium, hafnium, etc., which may cause a deviation of the final chemistry of the castings from the required specifications.
Investment castings are removed from the mold by breaking the molds and sometime by leaching the molds in hot caustic bath followed by grit blasting. These steps additionally increase the cost of production.
B. Ceramic-Mold Processes
If long-wearing, accurate castings of tool steel, cobalt alloys, titanium, or stainless steel are desired, ceramic molds are often used instead of sand molds.
The processes use conventional patterns of ceramic, wood, plastic, or metal such as steel; aluminum and copper set in cope and drag flasks. Instead of sand, a refractory slurry is used. This is made of a carefully controlled mixture of ceramic powder with a liquid catalyst binder (an alkyl silicate). Various blends are used for specific metal castings. Ceramic molds are used only one time and are expensive.
There is a need for improving the molding of various metallic alloys such as nickel, cobalt and iron based superalloys, nickel aluminides, stainless steel alloys, titanium alloys, titanium aluminide alloys, zirconium and zirconium base alloys. Metallic superalloys of highly alloyed nickel, cobalt, and iron based superalloys are difficult to fabricate by forging or machining. Moreover, conventional investment molds and ceramic molds are used only one time for fabrication of castings of metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, titanium alloys and titanium aluminide alloys. This increases the cost of production.
The term superalloy is used in this specification in its conventional sense and describes the class of alloys developed for use in high temperature environments and typically having a yield strength in excess of 100 ksi at 1000xc2x0 F. Nickel base superalloys are widely used in gas turbine engines and have evolved greatly over the last 50 years. As used herein the term superalloy will mean a nickel base superalloy containing a substantial amount of the xcex3xe2x80x2 (gamma prime) (Ni3Al) strengthening phase, preferably from about 30 to about 50 volume percent of the gamma prime phase. Representative of such class of alloys include the nickel base superalloys, many of which contain aluminum in an amount of at least about 5 weight % as well as one or more of other alloying elements, such as titanium, chromium, tungsten, tantalum, etc. and which are strengthened by solution heat treatment. Such nickel base superalloys are described in U.S. Pat. No. 4,209,348 to Duhl et al. and U.S. Pat. No. 4,719,080 incorporated herein by reference in their entirety. Other nickel base superalloys are known to those skilled in the art and are described in the book entitled xe2x80x9cSuperalloys IIxe2x80x9d Sims et al., published by John Wiley and Sons, 1987, incorporated herein by reference in its entirety.
Other references incorporated herein by reference in their entirety and related to superalloys and their processing are cited below:
xe2x80x9cInvestment-cast superalloys challenge wrought materialsxe2x80x9d from Advanced Materials and Process, No. 4, pp. 107-108 (1990).
xe2x80x9cSolidification Processingxe2x80x9d, editors B. J. Clark and M. Gardner, pp. 154-157 and 172-174, McGraw-Hill (1974).
xe2x80x9cPhase Transformations in Metals and Alloysxe2x80x9d, D. A. Porter, p. 234, Van Nostrand Reinhold (1981).
Nazmy et al., The effect of advanced fine grain casting technology on the static and cyclic properties of IN713LC, Conf: High temperature materials for power engineering 1990, pp. 1397-1404, Kluwer Academic Publishers (1990).
Bouse and Behrendt, Mechanical properties of Microcast-X alloy 718 fine grain investment castings, Conf: Superalloy 718: Metallurgy and applications, Publ:TMS pp. 319-328 (1989).
Abstract of U.S.S.R. Inventor""s Certificate 1306641, published Apr. 30, 1987.
WPI Accession No. 85-090592/85 and Abstract of JP 60-0644 (KAWASAKI), published Mar. 4, 1985.
WPI Accession No. 81-06485D/81 and Abstract of JP 55-149747 (SOGO), published Nov. 21, 1980.
Fang, J; Yu, B, Conference: High Temperature Alloys for Gas Turbines, 1982, Liege, Belgium, Oct. 4-6, 1982, pp. 987-997, Publ: D. Reidel Publishing Co., P.O. Box 17, 3300 AA Dordrecht, The Netherlands (1982).
Processing techniques for superalloys have also evolved as evident from the following references incorporated herein by reference in their entirety. Many of the newer processes are quite costly.
U.S. Pat. No. 3,519,503 describes an isothermal forging process for producing complex superalloy shapes. This process is currently widely used, and as currently practiced requires that the starting material be produced by powder metallurgy techniques. The reliance on powder metallurgy techniques makes this process expensive.
U.S. Pat. No. 4,574,015 deals with a method for improving the forgeability of superalloys by producing overaged microstructures in such alloys. The gamma prime phase particle size is greatly increased over that which would normally be observed.
U.S. Pat. No. 4,579,602 deals with a superalloy forging sequence that involves an overage heat treatment.
U.S. Pat. No. 4,769,087 describes another forging sequence for superalloys.
U.S. Pat. No. 4,612,062 describes a forging sequence for producing a fine grained article from a nickel base superalloy.
U.S. Pat. No. 4,453,985 describes an isothermal forging process that produces a fine grain product.
U.S. Pat. No. 2,977,222 describes a class of superalloys.
Since, the introduction of titanium and titanium alloys in the early 1950""s, these materials have found widespread uses in aerospace, energy, and chemical industries. The combination of high strength-to-weight ratio, excellent mechanical properties, and corrosion resistance makes titanium the best material for many critical applications. Titanium alloys are used for static and rotating gas turbine engine components. Some of the most critical and highly stressed civilian and military airframe parts are made of these alloys.
The use of titanium has expanded in recent years from applications in food processing plants, from oil refinery heat exchangers to marine components and medical prostheses. However, the high cost of titanium alloy components may limit their use. The relatively high cost is often fabricating costs, and, usually most importantly, the metal removal costs incurred in obtaining the desired end-shape. As a result, in recent years a substantial effort has been focused on the development of net shape or near-net shape technologies such as powder metallurgy (PM), superplastic forming (SPF), precision forging, and precision casting. Precision casting is by far the most fully developed and the most widely used net shape technology. Titanium castings present certain advantages. The microstructure of as-cast titanium is desirable for many mechanical properties.
The properties of titanium castings are generally comparable to wrought products in all respects and quite often superior. Properties associated with fatigue crack propagation and creep resistance can be superior to those of wrought products. As a result, titanium castings can be cost competitive with the forged and machined parts in many demanding applications. Titanium undergoes (alpha+beta) to beta allotropic phase transformation at a temperature range of 705xc2x0 C. to 1040xc2x0 C. well below the solidification temperature of the alloys. As a result, the cast dendritic beta structure is eliminated during the solid state cooling stage, leading to an (alpha+beta) platelet structure similar to typical wrought alloy. Further, the as-cast microstructure can be improved by means of post-cast cooling rate changes and subsequent heat treatment
Titanium castings respond well to the process of elimination of porosity of internal casting defects by hot isostatic pressing (HIP). Both elimination of casting porosity and promotion of a favorable microstructure improve mechanical properties. However, the high reactivity of titanium, especially in the molten state, presents a special challenge to the foundry. Special, and sometimes relatively expensive, methods of melting, mold making, and surface cleaning may be required to maintain metal integrity.
Lost wax investment molding was the principal technology that allowed the proliferation of production of titanium casting. The adaptation of this method to titanium casting technology required the development of ceramic slurry materials having minimum reaction with the extremely reactive molten titanium.
The titanium casting industry is still in its early stage of development. Because of highly reactive characteristics of titanium with ceramic materials, expensive mold materials (yttrium, throe and zircon) are used to make investment molds for titanium castings. The titanium castings develop a contaminated surface layer due to reaction with hot ceramic mold and molten titanium. This surface layer needs to be removed by some expensive chemical milling in acidic solutions containing hydrofluoric acid. Strict EPA regulations have to be followed to pursue chemical milling.
For example, U.S. Pat. No. 5,630,465 to Feagin discloses ceramic shell molds made from yttria slurries, for casting reactive metals. This patent is incorporated herein by reference.
The use of graphite in investment molds has been described in U.S. Pat. Nos. 3,241,200; 3,243,733; 3,256,574; 3,266,106; 3,296,666 and 3,321,005 all to Lirones and all incorporated herein by reference. U.S. Pat. No. 3,257,692 to Operhall; U.S. Pat. No. 3,485,288 to Zusman et al.; and U.S. Pat. No. 3,389,743 to Morozov et al. disclose carbonaceous mold surface utilizing graphite powders and finely divided inorganic powders termed xe2x80x9cstuccosxe2x80x9d and are incorporated herein by reference.
U.S. Pat. No. 4,627,945 to Winkelbauer et al., incorporated herein by reference, discloses injection molding refractory shroud tubes made from alumina and from 1 to 30 weight percent calcined fluidized bed coke, as well as other ingredients. The ""945 patent also discloses that it is known to make isostatically-pressed refractory shroud tubes from a mixture of alumina and from 15 to 30 weight percent flake graphite, as well as other ingredients.
It is an object of the invention to cast alloys in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to cast nickel, cobalt and nickel-iron base superalloys in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to cast nickel aluminide alloys in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to cast stainless steels in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to cast titanium and titanium alloys in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to cast titanium aluminides in isotropic graphite molds coated with pyrolytic graphite.
It is another objective of the present invention to cast zirconium and zirconium aluminide alloys in isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
It is another objective of the present invention to cast aluminum matrix composites reinforced with a high volume fraction of particulates and/or whiskers of one or more of compounds such as silicon carbide, aluminum titanium carbide and titanium diboride in an isotropic graphite mold with the mold cavity coated with pyrolytic graphite.
It is another object of the present invention to provide isotropic graphite molds with the mold cavity coated with pyrolytic graphite.
These and other objects of the present invention will be apparent from the following description.
This invention relates to a process for making various metallic alloys such as nickel, cobalt and iron based superalloys, stainless steel alloys, titanium alloys, titanium aluminide alloys, zirconium alloys and zirconium aluminide alloys as engineering components by vacuum induction melting of the alloys and subsequent casting of the melt in graphite molds under vacuum. More particularly, this invention relates to the use of high density high strength isotropic graphite molds with the mold cavity having been coated with pyrolytic graphite. The pyrolytic graphite is made via the chemical vapor deposition (CVD) technique to have very high purity (containing negligible trace elements).
The invention relates to pyrolytic graphite coating on bulk isotropic graphite that acts as the main body of the mold. Pyrolytic graphite (PG) is a unique form of graphite manufactured by decomposition of a hydrocarbon gas at very high temperature in a vacuum furnace. The result is an ultra-pure product that is near theoretical density and extremely anisotropic. The process used to form pyrolytic graphite is known as the chemical vapor deposition (CVD) technique. The chemical vapor deposition is carried out by the decomposition of low molecular weight hydrocarbon gases at a temperature of 1700xc2x0-2200xc2x0 C. The pyrolytic graphite deposited is annealed at temperatures in excess of 2600xc2x0 C. and pressures in excess of 5 torr.
In particular the invention relates to a method of making cast shapes of a metallic alloy, comprising the steps of:
melting the alloy to form a melt under vacuum or partial pressure of inert gas;
pouring the melt of the alloy into the cavity of a composite mold which is made essentially of isotropic graphite having a machined mold cavity, wherein the surface of the mold cavity is coated with a pyrolytic graphite coating; and
solidifying the melted alloy into a solid body taking the shape of the mold cavity.
Typically, the xe2x80x9ccxe2x80x9d direction of the pyrolytic graphite coating is perpendicular to the wall of the mold cavity and the pyrolytic graphite coating has a thickness between 0.1 to 5 mm. Also, typically the pyrolytic graphite has the following physical properties:
density of at least 2.1 gm/cc,
porosity of at most 1%,
compressive strength in the xe2x80x9ccxe2x80x9d direction of at least 65,000 psi at room temperature, and flexural strength in the xe2x80x9ccxe2x80x9d direction of at least 20,000 psi at room temperature.
Typically, the pyrolytic graphite has a density between 2.15 and 2.25 grams/cc and compressive strength between 65,000 psi and 70,000 psi and porosity less than 1%.
Attractive features of pyrolytic graphite include the following:
Chemically Inert
High Purity
Stable to 3000xc2x0 C.
Impermeable
Directional Electrical and Thermal Characteristics
Self-Lubricating
Nondusting
To construct a typical composite mold of the present invention, a mold with split halves is fabricated out of a high density isotropic graphite by machining a mold cavity of the required design into the graphite. Subsequently, the mold cavity is coated with a coating of anisotropic pyrolytic graphite via a chemical deposition process (CVD).
The CVD process parameters are controlled such that the coating is formed with the xe2x80x9ccxe2x80x9d direction of the graphite structure lying perpendicular to the mold wall surface. The xe2x80x9ccxe2x80x9d direction is at right angles to the basal planes of the graphite structure consisting of layers of carbon atoms arranged in a precise hexagonal pattern. Anisotropic pyrolytic graphite uniformly deposited over the surface of the mold cavity with xe2x80x9ccxe2x80x9d direction perpendicular to the mold wall offers the following advantages:
very high compressive strength,
extremely low thermal conductivity, and
an extremely dense and impervious coating.
Moreover, the above described composite molds, i.e., isotropic graphite molds coated with pyrolytic graphite, can be used to fabricate castings of superalloys, stainless steels, titanium alloys, titanium aluminides, nickel aluminides and zirconium alloys with improved quality and superior mechanical properties compared to castings made by a conventional investment casting process.
The molds can be used repeatedly many times thereby reducing significantly the cost of fabrication of castings compared to traditional processes.
Near net shape parts can be cast, eliminating subsequent operating steps such as machining.
As discussed above, the composite mold is made by a process including machining a cavity into a monolithic block of isotropic graphite and then coating at least the surface of the cavity with pyrolytic graphite. In the alternative, the isotropic graphite substrate can be initially molded to have the cavity and then have at least the surface of the cavity coated with pyrolytic graphite.
If desired, the composite mold may include a first substrate layer, a second substrate layer located over the first substrate layer and defining a cavity, and a layer of pyrolytic graphite coating at least the cavity of the second substrate layer. The second substrate layer would consist essentially of isotropic graphite. The first substrate layer may be made of any material which does not significantly interfere with operation of the mold. For example, a potential material for the first substrate layer may be extruded graphite.
Construction of composite graphite molds according to the present invention is more economical than the expensive process of fabricating a mold by machining a cavity into a monolithic block of pyrolytic graphite formed by a CVD process.
The mold of an isotropic graphite substrate coated with pyrolytic graphite would be more long lasting and perform better than a mold made of an extruded graphite substrate coated with pyrolytic graphite.